Highly loaded flourescent lamp particularly for dc operation

ABSTRACT

A highly loaded fluorescent lamp in which the discharge is confined to an inner vitreous tube within a sealed outer envelope. The tube is open at the ends and internally phosphor coated. Lower temperature control of the mercury vapor is achieved, together with higher electrical efficiency, particularly when a noncircular inner tube, such as an oval or grooved tube, is used to decrease the diffusion length of the discharge. The inner tube may be thin-walled because the pressure is the same on both sides. In DC operation, cataphoretic pumping of mercury vapor from the anode towards the cathode is largely overcome by gas flow in the zone between inner tube and outer envelope.

United States Patent [72] Inventor William H. Lake Novelty, Ohio 211 App]. No. 801,612 [22] Filed Feb. 24, 1969 [45] Patented Nov. 2, 1971 [73] Assignee General Electric Co.

[54] HIGHLY LOADED FLOURESCENT LAMP PARTICULARLY FOR DC OPERATION 5 Claims, 6 Drawing Figs.

[52] US. Cl 313/109, 313/185, 313/204, 313/225 [51] Int. Cl ..l-l0lj 61/20, H01 j 61/44 [50] Field ofSearch 313/27, 33, 109, 204, 228, 225,185, 205

[56] References Cited UNITED STATES PATENTS 1,547,766 7/1925 Lederer 313/33 2,172,839 9/1939 Francis et al.. 313/225 X 2,2611 18 12/1941 Marden 313/225 X 2,561,898 7/1951 Willoughby 313/205 X 2,950,410 8/1960 Lemmers et a1. 313/204 X 3,121,T83 .2/1'954 Swanson 3,435,272 3/1969 Green ABSTRACT: A highly loaded fluorescent lamp in which the discharge is confined to an inner vitreous tube within a sealed outer envelope. The tube is open at the ends and internally phosphor coated. Lower temperature control of the mercury vapor is achieved, together with higher electrical efficiency, particularly when a noncircular inner tube, such as an oval or grooved tube, is used to decrease the diffusion length of the discharge. The inner tube may be thin-walled because the pressure is the same on both sides. In DC operation, cataphoretic pumping of mercury vapor from the anode towards the cathode is largely overcome by gas flow in the zone between inner tube and outer envelope.

PATENTEDunvz I97! lnvn 'tor: WiLLiam HLake His A iiio neg HIGHLY LOADED FLOURESCENT LAMP PARTICULARLY FOR DC OPERATION BACKGROUND OF THE INVENTION The invention relates to low-pressure mercury vapor discharge lamps such as elongated tubular fluorescent lamps. It is particularly concerned with mercury vapor pressure control in highly loaded lamps and in lamps especially designed for operation on direct current or on pulsating current having a large direct current component.

In designing fluorescent lamps, it is usually desirable to increase the power input or loading without decreasing efficiency. However when the current density in a fluorescent lamp is increased, the electron temperature decreases and this results in a decrease in the efficiency of generation of 2,537 A. resonance radiation and a consequent drop in lamp efficiency. The electron temperature is a measure of the average kinetic energy acquired by the electrons due to the voltage gradient along the discharge. It is indicative of the fraction of electrons in the plasma having sufficient energy to raise mercury atoms with which they collide from their lowest energy level to a higher level from which they can drop back and emit radiant energy in the process. Thus at the same electron density, the lower the electron temperature, the fewer the photons of ultraviolet resonance radiation produced and the less the amount of visible radiation emitted by the phosphor.

In a low-pressure discharge lamp, the discharge is wall-stabilized, that is, the discharge is not constricted but extends up to the walls which form the boundary where electrons and ions recombine. The average distance which electrons and ions must travel to reach the walls of the lamp where they can recombine is known as the diffusion length. By decreasing the diffusion length, the rate of loss of electrons and ions to the walls is increased. An increase in the voltage gradient along the discharge then takes place in order to compensate for the higher rate of loss. The greater gradient means a higher electron temperature and higher efficiency together with higher wattage and loading of the lamp. One way of decreasing the diffusion length of the discharge is to use an envelope of noncircular cross section, for instance an oval cross section tube as in U.S. Pat. No. 2,482,421, Lemmers, or a grooved or kidney-shaped cross section tube as in U.S. Pat. No. 2,915,664, Lemmers. However envelopes of cross section other than circular tend to be weak against implosion by atmospheric pressure. For this reason they must be made with care and generally require a greater wall thickness which means a heavier and more expensive product.

The operation of elongated tubular fluorescent lamps on direct current presents the additional problem of electrophoresis which is the tendency of the discharge to pump or displace mercury vapor from one electrode towards the other. When the starting gas is argon, the phenomenon is cataphoresis and mercury pumped from the anode towards the cathode. Over a period of operation, mercury accumulates at the cathode end and the anode end may become dark due to mercury starvation. Cataphoresis is particularly pronounced and objectionable in relatively long tubular fluorescent lamps and it has made DC operation of such lamps impractical notwithstanding the advantages available from DC operation through the use of solid state circuitry. While the use of small mercury feedback passages as taught in my U.S. Pat. No. 3,l 17,248 is practical with specially shaped fluorescent lamps such as panel lamps and U-shaped lamps where the discharge channel is folded back upon itself so that the cathode region comes into physical proximity to the anode region, they are not practical for long straight tubular lamps.

SUMMARY OF THE INVENTION The objects of the invention are to provide elongated fluorescent lamps achieving lower temperature control of the mercury vapor together with higher electrical efficiency and in a form particularly adapted to counter cataphoretic mercury depletion when used on direct current.

In accordance with the invention, an unsealed light-transmitting inner tube, suitably of glass, is provided within the conventional sealed light-transmitting outer envelope of a fluorescent lamp. The inner tube is open at both ends and preferably internally phosphor coated and extends from the region of one electrode to the other. The electrodes are located within the ends of the tube but are mounted on inleads sealed through the ends of the outer envelope. The inner tube is of a size and cross section sufficient to contain the discharge by wall stabilization while allowing substantial circulation of ionizable medium in the intermediate zone between tube wall and envelope wall. This arrangement achieves lower temperature control of the mercury vapor than would be obtained with the same current flow through the inner tube in the absence of the outer envelope.

Higher electrical efficiency at high loading may be achieved by using an inner tube of noncircular cross section, for instance an oval or grooved tube in which the diffusion length of the discharge is decreased as the ratio of the perimeter to cross-sectional area is increased. Since the pressure is substantially the same on both sides of the inner tube, it may be thin walled and lightweight.

A feature of the invention particularly advantageous in DC operation of the lamp is the circulation of the fill gas such as V neon, argon, krypton or mixtures thereof in the intermediate zone between the inner tube and the outer envelope. The circulation is caused by electrophoresis which tends to build up a pressure differential in the fill gas from anode to cathode, that is in the opposite direction to the pressure differential in the mercury vapor. Since the pressure of the fill gas is much higher than that of the mercury vapor, being measured in millimeters as against microns, the circulation of the till gas causes the mercury vapor to be swept along with it so that the mercury pressure differential from cathode to anode is greatly decreased or substantially eliminated.

DESCRIPTION OF DRAWINGS FIG. 1 is a side view of a discharge lamp embodying the invention using a round section inner tube.

FIG. 2 is a cross section of the lamp of FIG. 1 on section line 2-2.

FIG. 3 is a side view of a lamp embodying the invention in which the inner tube is of oval cross section.

FIG. 4 is a cross section through the lamp of FIG. 3 on section line 4-4.

FIG. 5 is a side view of a lamp embodying the invention in which the inner tube has indented walls forming a series of spaced longitudinally extending grooves.

FIG. 6 is a cross section through the lamp of FIG. 5 on section line 6-6.

In each of FIGS. 1, 3 and 5, the lamp has been shortened by cutting out a central portion and the right-hand portion has been rotated on the longitudinal axis of the lamp relative to the left-hand portion.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, an improved highly loaded fluorescent lamp embodying the invention comprises an elongated outer glass envelope 1 of substantially uniform diameter that is sealed at both ends and provided with suitable end caps or bases 2,3. Oppositely disposed mount assemblies 4,5 and electrodes 6,7 are sealed in the end of the envelope through the usual stem presses. As illustrated, each electrode conventionally comprises a coiled coil filament provided with an overwind coated with alkaline earth oxides or other electronemitting substance. However the electrode intended as anode in direct current operation of the lamp need not be a filamentary structure and alternatively may be a plan metal member or disc as shown at 7a in FIG. 3. The electrodes are connected by inleads to the terminals of the bases 2,3. The envelope is provided with an ionizable filling comprising a small quantity of mercury indicated by a droplet 8 which is in excess of the quantity required to provide a mercury vapor pressure in the range of 2 to I microns throughout the lamp envelope during normal operation. The lamp filling also comprises an inert gas for starting purposes, suitably argon in the pressure range of 2 to 3 millimeters of mercury.

In accordance with the invention, an inner glass tube 9 extends coaxially within the outer envelope from the vicinity of one electrode to the other. Preferably the electrodes 6, 7 penetrate a short distance into the ends of the tube. The inner tube may be supported within the outer envelope in various ways. One suitable arrangement is to provide three extra support leads 11 in mounts 4, respectively. These support leads are embedded in the stem press whereby they are positively anchored and have booked ends 12 which resiliently engage the edge of the tube 9. This results in a flexible mounting which constrains the inner bulb and at the same time protects the entire lamp structure from shock and vibration.

By way of example, one lamp constructed according to FIG. 1 utilized a T-l 2 (1% inches nominal diameter) glass tube for the outer envelope and a T-IO (I la inches nominal diameter) glass inner tube. The inner tube is coated internally with a phosphor l3 responsive to the UV radiation produced by the discharge, suitably a halophosphate-type phosphor. Surprisingly the lamp operated with a lower mercury pressure than would a conventionally constructed T-l2 lamp operating at comparable current. The explanation for this unexpected condition appears to be that a redistribution of the temperature developed in the lamp is caused by circulation of the argon fill gas. Electrophoretic action would tend to move the argon from cathode to anode within the inner tube. This causes a circulation to be set up wherein the argon returns to the cathode in the intermediate zone between the inner tube and the outer envelope and the mercury vapor is swept along with the argon. The lamp illustrated in FIG. I was operated with a discharge current of X 1.5 amperes at a temperature rise of about 16 over ambient. This is no greater than a conventional lamp of the size of the outer envelope would exhibit at an operating current of 430 milliamperes.

A lamp structure using an inner glass tube or channel wherein the discharge is confined and which is located within an outer envelope which alone is subject to atmospheric pressure lends itself admirably well to the use of an inner tube of noncircular cross section for decreasing the diffusion length of the discharge. An inner tube must of course be chosen having sufficient cross-sectional area that the discharge will go through it and not through the space between the tube wall and the outer envelope wall. In FIGS. 3 and 4, an inner tube 14 is used which is of oval cross section except at the ends 15 which overreach the electrodes. The tube is internally coated with phosphor indicated at 13. In the lamp of FIG. 3, the outer envelope 1a may be a T-l4 tube (nominal 1-% inches diameter) and the inner tube may be a T-lO (I% inches nominal diameter) tube flattened to the oval cross section illustrated. For an oval tube to withstand atmospheric pressure, it must be relatively thick walled and the narrow edges must be prestressed according to the teachings of U.S. Pat. No. 2,482,421, Lemmers, Flat-Tube Electrical Device." In the present construction, the pressure is the same on both sides of the inner tube so that it may be made thin walled and no prestresssing of the narrow edges is required.

In FIGS. 5 and 6, the inner tube 16 has dents l7 alternating on opposite sides forming a series of spaced longitudinally extending grooves according to the teachings of U.S. Pat. No. 2,915,664, Lemmers, Tubular Electric Lamp." This configuration has the advantage of decreasing the diffusion length of the discharge and of also lengthening the effective discharge path occuring within a given length of lamp. By placing the grooved tube within an outer envelope, the requirement for sufficient strength to resist atmospheric pressure is eliminated and a thin-walled inner tube may be used which is both lighter and less expensive. In addition, the convolutions of the inner tube cannot collect dirt and the smooth outer envelope lacleans readily like a conventional tubular lamp. The inner tube is intemall coated with phosphor l3.

The lamps 0 FIGS. 3 and 4 having an oval inner tube and that of FIGS. 5 and 6 having grooved inner tube are particularly suitable for use as highly loaded lamps on DC operation. The inner bulb then serves to decrease the diffusion length of the discharge to the phosphor-coated wall making possible higher efficiency at high loading. At the same time the structure permits circulation of the fill gas to counter the cataphoretic effect and prevent mercury depletion of the lamp. Since the outer envelope that withstands atmospheric pressure is of circular section, it has adequate strength without requiring any greater thickness than the conventional fluorescent lamp, and the inner tube may be thin-walled since it does not withstand any pressure. This combination of features makes possible a lamp for operation on direct current which weights no more and which has a loading and efficiency at least as good as the single envelope highly loaded lamps used on alternating current.

What I claim as new and desire to secure by Letters Patent of the United States is:

I. A low-pressure electric discharge lamp comprising a sealed elongated vitreous tubular envelope containing an ionizable medium comprising a small quantity of mercury exerting in operation a partial pressure in the range of 2 to 10 microns and an inert gas at a pressure of a few millimeters of mercury, electrodes sealed into opposite ends of said envelope for sustaining an electric discharge through said medium, an elongated unsealed vitreous inner tube mounted within said envelope and extending from the region of one electrode to the other, said inner tube being unobstructed at its ends and being of a size and having a cross section sufficient to contain the discharge by wall stabilization while allowing substantial circulation of inert gas in the space between the tube wall and the envelope wall, said inner tube being coated internally with a phosphor.

2. A lamp as in claim 1 wherein said envelope is substantially circular in cross section to better withstand atmospheric pressure and said inner tube is noncircular in cross section in order to decrease the diffusion length of the discharge to the tube wall.

3. A lamp as in claim 2 wherein said inner tube is oval in cross section.

4. A lamp as in claim 2 wherein said inner tube has reentrant sections forming short grooved portions extending longitudinally and alternating on opposite sides.

5. A lamp as in claim 1 wherein the inert gas includes a substsntial proportion of argon, neon, or krypton. 

2. A lamp as in claim 1 wherein said envelope is substantially circular in cross section to better withstand atmospheric pressure and said inner tube is noncircular in cross section in order to decrease the diffusion length of the discharge to the tube wall.
 3. A lamp as in claim 2 wherein said inner tube is oval in cross section.
 4. A lamp as in claim 2 wherein said inner tube has reentrant sections forming short grooved portions extending longitudinally and alternating on opposite sides.
 5. A lamp as in claim 1 wherein the inert gas includes a substantial proportion of argon, neon, or krypton. 